The present invention relates to documents having retroreflective properties, and more particularly to documents such as labels, designs, and decals which have a retroreflective surface which is receptive to printed indicia.
Retroreflective surfaces are used in a variety of applications where a strong reflective return of directed light is desired. Such applications include labels, designs, decals, barrier markers, and the like. These surfaces are typically comprised of a substrate which has coated on at least one surface thereof a layer of glass microspheres which are adhered to the substrate with an adhesive or binder. Each glass microsphere acts as a reflecting lens, returning incident light which impinges thereupon within a narrow angle cone. When light is directed toward such a surface at an angle of approximately 90xc2x0, its is strongly reflected back in the direction of the light source and appears to a viewer as a strong and independent light source emanating from the surface.
This retroreflective technology has been developed and adapted for many uses, and commercial retroreflective products are available from numerous sources including 3M Corp. under the trademark Scotchlite(copyright), Reflexlite North America, and Printmark Industries. A number of retroreflective products use three-dimensional microprisms which are embossed into the surface of a transparent film substrate instead of bonding microspheres to the surface.
Documents such as labels and decals have a variety of uses including imparting information about a product to a user thereof. In many instances, an adhesive is used to secure the labels or decal to an underlying surface. Where the document is used in an outdoor environment, it is important that the document, and any printed information thereon, is resistant to sunlight, moisture, abrasion, and other environmental factors.
Also, in many applications, information is printed onto a label or decal surface prior to use. The information may be repetitive. That is, the information which is printed may be the same (repetitive) for a series of labels. Typically, such repetitive information is provided by conventional printing processes such as flexography, gravure, letterpress, lithography, and screen printing. Additionally, at least a portion of the information which is printed may be non-repetitive. That is, the information which is printed is unique to each individual label. Typically, non-repetitive information is provided using impact printers such as dot matrix printers, or non-impact printers such as laser, thermal transfer, or ink jet imaging, all under computer control.
An example of repetitive information printed on a label would be generic information concerning the manufacturer of the label, the name of the product, etc. An example of non-repetitive information would be a unique serial or product number or bar code which was unique to an individual label.
One common use for such labels are annual renewal stickers for automobile and truck license plates. These labels have a generic or repetitive portion common to all of the labels which typically includes the name of the state and the year and/or month of expiration. The labels also include an identification number or series of letters and numbers which is specific to the vehicle. Such unique identification numbers are typically applied by a dot matrix, laser, thermal transfer, or ink jet printer. All of the images and information on such labels, including the unique vehicle identification numbers, must be able to withstand exposure to the environment (weather, light, abrasion, solvents and cleaners, etc.) for at least one year.
There has been a problem for such vehicle labels in that the laser or ink jet printed non-repetitive information, and even the conventionally applied repetitive information, does not adhere well to the surface of typical retroreflective substrates. Generally, glass microspheres are bonded to the substrate by a polymeric binder and protrude from the binder by approximately one-half of the diameter of the spheres. In many instances, a polymeric top coat will be applied over the binder and microspheres. The polymeric binder and/or top coat may be any of a number of known resins including alkyds, polyurethanes, polyamides, polyesters, and the like. Such binders are typically chosen for their ability to bond the microspheres to the substrate, not for their ability to receive inks or toners. And, the top coat polymers are typically chosen for optical transparency, not for their ability to receive inks and toners. Thus, the inks or toners which are applied to the retroreflective surface of the substrate tend to flake or prematurely peel away.
One attempted solution to this problem has been to apply coatings having enhanced toner adhesion properties over the retroreflective surface. To such coatings, the ink or toner is applied. However, heretofore such coatings have been at least somewhat opaque due to the presence of various fillers and/or pigments, and have interfered, at least to some degree, with the retroreflective surface properties of the label making the label less reflective.
While other adhesion enhancing coatings are known, they are typically coated onto cellulosic substrates which have very different surface properties than retroreflective surfaces. Examples of prior art adhesion enhancing coatings include U.S. Pat. Nos. 5,045,426, 4,510,225, 2,855,324, 3,130,064, 4,863,783, 4,071,362, 5,017,416, 5,219,641, and 5,698,296. However, such prior art coatings suffer from a number of drawbacks including excessive crosslinking of the polymers, lack of optical transparency, incompatibility with polymeric substrates, and an inability to apply such coatings using conventional printing techniques.
Accordingly, there remains a need in this art for a coating which can be readily applied over a retroreflective surface by printing techniques and which enhances the adhesion of toners and inks printed thereon while not substantially interfering with the retroreflective properties of the underlying substrate. Preferably, the coating would also provide resistance to degradation of the printed indicia from environmental sources including solvents, physical abrasion, detergents, water, and sun light.
The present invention meets those needs by providing a coating for a retroreflective document which renders the surface of the document receptive to toners and inks printed thereon while not substantially interfering with the retroreflective properties of the underlying substrate. That is, a coating of the present invention will not substantially reduce the retroreflective properties of the document. The term xe2x80x9creceptivexe2x80x9d as used herein refers to the ability of the coating to provide a surface to which inks and toners adhere or bond well to, thereby improving the quality and durability of the printed indicia. By xe2x80x9cnot substantially reduce,xe2x80x9d it is meant that the retroreflectivity of the substrate remains at least about 60%, and preferably at least about 75%, of its original value. The coating also provides resistance to degradation of the printed indicia from environmental sources including solvents, physical abrasion, detergents, water, and sun light, including those indicia which may be pre-printed on the retroreflective surface prior to the application of the coating of the present invention.
In accordance with one aspect of the present invention, a retroreflective document having a transparent ink and toner receptive coating thereon is provided. The coating consists essentially of a crosslinked acrylic acid resin which is optically transparent and does not substantially interfere with the retroreflective properties of the underlying sheet, which increase adhesion or bonding of toners and inks to the retroreflective sheet, and which is resistant to environmental attack. In one embodiment, the crosslinked acrylic acid resin is applied as an aqueous dispersion or emulsion consisting essentially of from about 50 to about 70 wt % acrylic acid resin, from about 2 to about 5 wt % of a crosslinking agent, optionally from about 0 to about 2 wt % of a surfactant, and the balance water.
In another embodiment, the toner and ink receptive coating is applied as a 100% solids (i.e., solvent-free) liquid consisting essentially of from about 50 to about 98 wt % of an acrylic acid ester monomer or oligomer having ethylenic unsaturation in its backbone, from about 2 to about 8 wt % of a photocatalyst, and from about 0.5 to about 2 wt % of a surfactant.
The present invention also provides a process for making a printable retroreflective document which includes the steps of providing a retroreflective substrate, printing a liquid, transparent toner and ink receptive coating consisting essentially of a crosslinkable acrylic acid resin and a crosslinking agent onto the substrate, and curing and crosslinking the toner and ink receptive coating. The retroreflective substrate may include a monolayer of microspheres bonded to the substrate or, alternatively, may include embossed or adhered microprisms on the substrate. The process may also include the step of printing indicia onto the surface of the toner and ink receptive coating. This printing step may use conventional inks, ink jet inks, thermal transfer inks, impact ribbon inks, or the printing step may use toner applied from a laser or other non-impact printing device.
In an alternative embodiment of the invention, a process for making a printable retroreflective document is provided and includes the steps of providing a reflective substrate, mixing microspheres with a liquid, transparent toner and ink receptive coating consisting essentially of a crosslinkable acrylic acid resin and a crosslinking agent onto the monolayer of microspheres to form a slurry, applying the slurry to the surface of the reflective substrate, and curing and crosslinking the toner and ink receptive coating. In yet another alternative embodiment, a process for making a printable retroreflective document is provided and includes the steps of providing a reflective substrate, applying a liquid, transparent toner and ink receptive coating consisting essentially of a crosslinkable acrylic acid resin and a crosslinking agent onto the reflective substrate, applying and embedding a monolayer of microspheres in the toner and ink receptive coating, and curing and crosslinking the coating.